Implantable pacemakers are coupled to the heart through a lead system. In operation, the pacemaker periodically provides an electrical output stimulus which is delivered to the complex impedance of the heart. The amount of energy delivered to the heart depends on the resistance of the lead and the heart tissue. It has been routine to measure this impedance or threshold at the time of implantation to permit optimizing the location of the pacing lead and to maximize longevity of the pacer. These acute measurements are made with an oscilloscope or, more frequently, with a special instrument called a pacing system analyzer or PSA.
Later, after the lead has "healed" into the heart tissue, the margin of capture and delivered energy are estimated by the physician. With the advent of output pulse programmable pacers, physicians have been able to adapt the pacer's output to the threshold requirements of the patient and thus prolong the longevity of the implanted device.
To assist the physician with followup care, modern pacers use internal circuitry to monitor the output pulse parameters and to telemeter this information to the physician via a programmer. This information is used in assessing the performance of the pacemaker and the associated lead.
A problem which is presented by this technology is a discrepancy between the measured values of lead current and delivered energy presented by the programmer, PSA and oscilloscope. These differences result primarily from different measurement methodologies.
Typically, prior art pacers place a series resistor in the output path of the lead current. During the delivery of a pacing stimulus, the voltage drop across the resistor is measured and telemetered out. This technique requires the use of a high precision resistor to reduce measurement errors. It also introduces a component whose failure can lead to an undesirable "no output" condition. Also, such a system is wasteful of output energy because of the inclusion of the measuring resistor.